The thick film dielectric structure provides for superior resistance to dielectric breakdown as well as a reduced operating voltage as compared to thin film electroluminescent (TFEL) displays, as exemplified by U.S. Pat. No. 5,432,015. The thick film dielectric structure also enhances the amount of charge that can be injected in to the phosphor film to provide greater luminosity than can be realized from TFEL displays. Full colour thick dielectric electroluminescent displays are described for example in the Applicant's PCT CA03/01567. These displays employ high luminance blue phosphor materials to directly illuminate blue sub-pixels and colour conversion materials to down-convert the blue light to red or green light for the red and green sub-pixels. While these thick film electroluminescent structures meet the luminosity and colour spectrum capability of cathode ray tube (CRT) based displays, the operating stability still falls short of that provided by CRTs. Therefore, improvements to thick film dielectric structures are desirable.
One area in which improvement is sought is the prevention of the degradation of the phosphor layers used in the thick film dielectric electroluminescent structures. In this regard, various materials have been proposed for use with certain types of phosphor materials. U.S. Pat. Nos. 5,496,597 and 5,598,059 disclose the use of aluminum oxide in conjunction with a terbium doped zinc sulfide for electroluminescent displays. Aluminum nitride has also been proposed for use with EL emitting layers of alkali earth chalcogen compounds in thin film electroluminescent devices as described in U.S. Pat. No. 4,975,338 and JP 02103893. Aluminum nitride has also been used as an insulating thin film layer in organic electroluminescent elements/displays as described in JP 08288069, JP 10092580, U.S. Pat. No. 6,146,225, U.S. Pat. No. 6,383,048 and U.S. Pat. No. 6,416,888 as well as a moisture barrier layer in EL elements as described in U.S. 2002/0079836 and U.S. 2002/0031688. Aluminum nitride as a ceramic substrate has also been proposed as disclosed in U.S. 2002/0177008.
U.S. 2002/0125821 discloses the use of aluminum nitride as a semiconductor material interposed between a conventional manganese activated zinc sulfide phosphor film and a thick film dielectric layer. The device is constructed by annealing the phosphor film, applying a layer of aluminum nitride on top of the annealed phosphor and then screen printing and sintering a thick film dielectric layer thereon such that the aluminum nitride is placed between the phosphor film and the thick film dielectric layer. Aluminum nitride layers are also disclosed art for use as barrier layers in thick dielectric electroluminescent displays. PCT CA02/01891 describes the use of aluminum nitride layers either above or below the phosphor layer in such electroluminescent devices.
Aluminum oxide barriers are disclosed in the prior art as a barrier layer for electroluminescent displays. For example Japanese patent application 2003-332081 discloses an aluminum oxide layer disposed between the thick dielectric layers and the phosphor layer in a thick dielectric electroluminescent device. In the disclosed device the aluminum oxide layer is not in contact with the phosphor layer, but rather a zinc sulfide layer is positioned between the aluminum oxide layer and the phosphor layer.
Aluminum oxide layers are also known to be used in organic electroluminescent devices where such layers are provided adjacent to a phosphor or substrate as described for example in U.S. Pat. Nos. 4,209,705, 4,751,427, 5,229,628, 5,858,561, 6,113,977, 6,358,632 and 6,589,674 as well as in U.S. 2003/0160247 and U.S. 2004/0115859.
While the aforementioned patents and patent applications may teach the use of certain insulator materials such as aluminum nitride in conjunction with conventional zinc sulfide phosphors within thin film or thick film electroluminescent displays, there remains a need to provide further improvements to thick film dielectric electroluminescent displays that will further improve the luminance of phosphors provided therein and also extend their operating life with minimal degradation.